docs: small wording fix in memory_allocation.rst

"uninitialized value" in C/C++ is not the same as "filled with random data":

In practice, on most implementation the content is not actually random.
Some implementations *may* zero-initialize malloc()-allocated memory, but AFAIK it's not something common, nor it's portable.
In most other cases, the value will be "whatever was in memory on that address previously", which in general is also far from random/pseudorandom.

Speaking in terms of standards, if I understand correctly, accessing uninitialized value may, or may not, cause Undefined Behaviour. It goes like this:
- 'uninitialized' variable has 'indeterminate' value.
- 'indeterminate value' may be (depending on implementation) either:
  - an 'unspecified value' - no UB, just value can be anything.
  - a 'trap representation' (only if type of variable is other than [signed or unsigned] char). Merely accessing such value results in 'undefined behaviour'.

See also:
https://stackoverflow.com/questions/9219971/read-before-write-is-undefined-with-malloced-memory
https://stackoverflow.com/questions/11962457/why-is-using-an-uninitialized-variable-undefined-behavior-in-c